1. Field of the Invention
The invention relates generally to the field of gantry positioning systems. More particularly, the invention relates to controlling and detecting skew in a gantry positioning system for semiconductor wafer transfer applications.
2. Discussion of the Related Art
Gantries in general face considerations resulting from dynamic or unbalanced loads that are transported across or along axes of the gantry. In applications related to silicon wafer fabrication processes, gantries are sometimes required to position robots that transport wafers between modules for performing fabrication steps on wafers. The robots serve to position wafers to a high degree of precision. For instance, robots are used to precisely position wafers within cassettes so as to place the wafers three pins or more pins. If the robot is even slightly misaligned, the robot may fail to transfer or place the wafer. However, the weight of the robot may also cause skew to develop on the gantry that affects alignment and ability to precisely position the wafers. In the worst case, skew may damage the robot or components on the gantry. Therefore, semiconductor wafer transfer gantries in particular face control problems arising in part from skew and misalignment.
As practiced in the known art, the control of gantry positioning systems is limited to using direct drive coupled systems that are unable to compensate for skew. For example, the known art provides for a gantry having gantry axes directly coupled together by a T gear box and/or miter gearbox. Such directly coupled gantries of the known art cannot compensate for deflections and misalignments due to dynamically driven loads. What is needed, therefore, is an approach to compensating for skew in a gantry.